The current apparatus to accomplish bore sight verification is a large assembly of two parts, the M154 Alignment Device and the Bore Sight Adapter, both known in the art. The Bore Sight Adapter is inserted into the mortar bore and rotated to level. The M154 is then assembled onto the dovetail of the Bore Sight Adapter. The user views the crosshairs inside the M154 collimator through the sight mounted on the weapon, and uses the micrometer knobs to align the crosshairs of the M154 and the weapon's sight. The user then reads the angle measured from the micrometer knob and compares the new, measured value to the standard value. If the measured value is the same as the standard value within tolerance, the mortar is considered to have a verified bore sight.
The current method and equipment has a number of limitations and disadvantages.
The Bore Sight Adapter uses a rubber o-ring to locate and hold the assembly level in the bore. This o-ring must maintain a coat of grease. If ungreased, the o-ring will tear when the Bore Sight Adapter is leveled. However, excessive grease is also an issue; with excessive grease, the o-ring will no longer hold, allowing the weight of the M154 and the Bore Sight Adapter to pull the unit out of level. If the o-ring falls down the bore of the mortar, no tool exists to retrieve it. Therefore, the mortar must be taken back to the depot for special maintenance to disassemble the weapon in order to extract the o-ring.
The Bore Sight Adapter uses a dovetail-style mount to secure the M154. However, there is no physical locator to force the M154 to be in the same place from use to use, creating the problem of repeatability in measurements. The level vial used to level the assembly is located at the top of the Bore Sight Adapter. In this location, it is difficult to read, and it is unprotected from impact and the elements.
To use the weapon, the mortar must be elevated to a point near the extreme limit of travel. Not only does this take valuable emplacement time and effort, it also presents a set of optically-related challenges. The M154 must be in the same optical plane as the sight telescope for proper operation. When on solid ground before firing, the base plate of the mortar is sitting on the ground and this is not an issue. If the users must verify bore sight after firing, the base plate has sunk into the ground, and it may no longer be possible to elevate the mortar to the proper elevation for verifying bore sight. In this case, the weapon must be relocated, laid, and bore sight must be verified again before allowed to fire, which would take several minutes.
The manufacturing tolerance stack-up from the o-ring to the end of the M154 is excessive. Tolerance stack-up is a phenomenon which occurs when the individual parts of a component are all manufactured within required specifications, but the resulting larger component is out of tolerance as a result of the variances of its components. For example, if a 12-inch (±0.5) bar is needed out of three 4-inch (±0.2) components, it is possible to have a 12.6-inch bar, which is out of tolerance, comprised of three 4.2-inch components, each of which is in tolerance.
The tolerance stack-up from the o-ring to the end of the M154 increases the angle tolerance, resulting in a large tolerance which is unacceptable in the artillery field. The size of the M154 and Bore Sight Adapter are also a potential hindrance due to the large protective case they are stored in. When loaded, the case is heavy and takes up a large amount of the valuable cargo area inside a vehicle.